ADVANCED EXPEEIMENTAL PHYSIOLOGY 369 



CHAPTER XXXV. 

 THE OPTICAL DEFECTS OF THE EYE. 



1. Myopia and Hypermetropia. The condition of the refractive 

 media of the eye when either hypermetropia or myopia are present 

 are conveniently tested by what is known as the shadow test. If one 

 take a concave mirror (such as that of an ophthalmoscope used for 

 the indirect method), and reflects the' light of a lamp at the side of 

 the subject into the pupil of the eye, on looking through the aperture 

 in the mirror the back of the eye is seen to be partially illuminated. 

 If the subject be emmetropic the amount of illumination is small, 

 and on tilting the mirror a little to the right or left a scarcely 

 perceptible movement of the light area may be seen in the opposite 

 direction of the tilt. The image of the lamp formed by the concave 

 mirror is the direct source of illumination of the subject's eye, 

 and this image moves to the right when the mirror is tilted to the 

 right, and in accordance with the inversion of the image on the retina 

 the illuminated area M 7 ill seem to pass to the left. The general impres- 

 sion that one obtains of the result of tilting the mirror on the emme- 

 tropic eye is that the illumination suddenly disappears. With the 

 hypermetropic eye the illuminated area is more distinct, as a large part 

 of it can *now be seen, and the passing of this area to the right or left 

 inversely to the tilting of the mirror to the left or right is clearly 

 visible. In the case of myopia the observer must be beyond the far 

 point of the eye and then will see an inverted image of the illuminate 

 area. As the result the apparent illuminated area will be an inversion 

 of the actual area. When therefore the mirror is tilted and the image 

 of the lamp passes across from right to left, the apparent movement will 

 be from left to right, so that the movement of the light on the retina 

 appears to be the same as the tilt of mirror. A small amount of 

 myopia cannot be made out by this method. 



EXPERIMENT. If subjects possessing the defects of myopia and 

 hypermetropia cannot be obtained, using the ophthalmoseopic mirror as 

 directed above, observe the movement of the light on the retinal screen 

 in KUhne's artificial eye adapted for these defects. Compare the actual 

 movement of the light on the screen with the apparent movement when 

 observing in front of the eye as above. 



2. Imperfections of the Eefracting Media, EntopticThenomena. (a) 

 Certain bright, cloudy appearances may be seen, which disappear after 

 blinking the eyelids. Wavy lines or speckled patches may appear after 



2A 



